Chap. XXVIII. CONCLUDING EEMAEKS. 415 



over the snow, may have become partially webbed from the 

 stimulus of widely extending their toes ; but it is more pro- 

 bable that the webbing, like the membrane between the toes 

 of certain pigeons, spontaneously appeared and was afterwards 

 increased by the best swimmers and the best snow-travellers 

 being preserved during many generations. A fancier who 

 wished to decrease the size of his bantams or tumbler-pigeons 

 would never think of starving them, but would select the 

 smallest individuals which spontaneously appeared. Quad- 

 rupeds are sometimes born destitute of hair and hairless 

 breeds have been formed, but there is no reason to believe 

 that this is caused by a hot climate. Within the tropics heat 

 often causes sheep to lose their fleeces ; on the other hand, 

 wet and cold act as a direct stimulus to the growth of hair ; 

 but who will pretend to decide how far the thick fur of arctic 

 animals, or their white colour, is due to the direct action of 

 a severe climate, and how far to the preservation of the best- 

 protected individuals during a long succession of genera- 

 tions 9 



Of all the laws governing variability, that of correlation is 

 one of the most important. In many cases of slight deviations 

 of structure as well as of grave monstrosities, we cannot even 

 conjecture what is the nature of the bond of connexion. But 

 between homologous parts — between the fore and hind limbs 

 — between the hair, hoofs, horns, and teeth — which are closely 

 similar during their early development and which are exposed 

 to similar conditions, we can see that they would be eminently 

 liable to be modified in the same manner. Homologous parts, 

 from having the same nature, are apt to blend together, and, 

 when many exist, to vary in number. 



Although every variation is either directly or indirectly 

 caused by some change in the surrounding conditions, we 

 must never forget that the nature of the organisation which 

 is acted on, is by far the more important factor in the result. 

 We see this in different organisms, which when placed under 

 similar conditions vary in a different manner, whilst closely- 

 allied organisms under dissimilar conditions often vary in 

 nearly the same manner. We see this, in the same modifica- 

 tion frequently reappearing in the same variety at long 



